An integrated optical directional coupler, formed by two parallel strip waveguides, can be used as a modulator by electrically switching the coupler between its crossover state and its through state. One way of doing this is to fabricate the coupler on an electrooptic material and apply a switching voltage which induces a mismatch (.DELTA..beta.) via the electrooptic effect. The rate at which the modulator can be switched depends primarily upon the capacitance of the electrodes used to apply the electric field. At present, switching rates in excess of 5 GHz have been achieved with very small devices employing waveguide and electrode spacings of about one micron.
While it is relatively easy to obtain waveguide spacings of one micron by using conformable mask exposure techniques wherein very close contact is maintained between the mask and the underlying substrate (i.e., separations of less than 1000 A), it has been much more difficult to achieve comparable spacing between electrodes using relatively thick glass masks. Particularly so, when the wavelength of the illuminating radiation is comparable to both the mask aperture and to the distance between the mask and the underlying substrate surface. Under these conditions, diffraction effects tend to limit the obtainable resolution.